The present invention pertains to coherence microscope performed through optical fiber bundles.
Coherent optical fiber bundles that preserve the relative ordering of fibers at their respective ends are commonly used for video imaging of internal structures during internal exploration and surgery. Such means enable a physician or surgeon to see the surface of internal structures using visible light by means of a video camera coupled to the fiber bundle.
A coherent fiber bundle is a bundle of optical fibers that are arranged such that they remain parallel to each other the length of the bundle, in such a way that the nearest neighbor fibers of a particular fiber on one end of the bundle are also the neighbors on the other end. Each of the fibers in the bundle is a waveguide that conducts light from one end of the fiber to the other. The fiber typically consists of a high refractive index core, and a low refractive index cladding, and the light is confined to the core by total internal reflection. By focusing an image onto one end of the bundle, various points on the image are conducted through their respective fibers to the other end of the bundle. Because the bundle is “coherent,” these points of the image emerge from the bundle in the same relative positions as they were on the incident end. The image emerging from the bundle can be further imaged onto a sensor, where it can be displayed on a television screen for visualization, for example. This is the basis for a typical fiber bundle endoscope.
FIG. 1 is a simplified schematic of an endoscope 100 as employed in the current art. A light source 102, such as a halogen light bulb, produces white light 106, which is used to illuminate the object being studied. This white light is imaged into the proximal end of a fiber bundle 108 using a lens 110 and is conducted to the object through the bundle. The light is carried in the cores 112 (shaded, in FIG. 1) of each fiber in the bundle, thus the light carried in each core remains largely separated for the length of the bundle. The illumination emerges from the distal end of the bundle and is focused by a exit optics 114 (such as second lens 114) onto the object 104. The light scatters off of the object and is recollected by exit optics 114 back into bundle 108. Part of the scattered field is coupled back into the cores 112 in the bundle, where the image is conducted from the distal back to the proximal end of the bundle. The separation between the cores keeps the light from the cores from mixing together, at least substantially, so that each core conducts essentially one picture element of the resulting image. The lens 110 then focuses the light emerging from the cores of the bundle, via a beamsplitter 118, onto a sensor 116, where pixels on the sensor detect the light from the cores.
While the fiber bundle is depicted schematically in FIG. 1 as a short rod, in reality it is typically long and flexible, although rigid designs also exist. The typical diameter of a bundle is from 0.5-5.0 mm, and the length is 10 cm or longer, up to several meters. The fiber bundle can be threaded through internal luminal structures such as within the vascular system and the gastrointestinal, urinary, or respiratory tracts. These bundles are also used to visualize regions beneath organs that a rigid endoscope would be unable to reach. A bundle can contain from several hundred to 100,000 individual fibers, so the image can be quite detailed. The fiber bundle can be integrated into a catheter with surgical instruments, so that image-guided surgery is commonplace with fiber bundles. Fiber bundles are made of durable silica or plastics and therefore can tolerate tight bends without breaking.
It is, however, desirable to visualize not only the readily imaged surface but also the subsurface structure (to, say, 1-2 mm depth) of internal tissues for various reasons, including detecting cancers in epithelial tissues (which form 85% of diagnosed cancers). Unfortunately, visible light video imaging is unsuitable for this, because of the very shallow penetration of visible light into epithelial tissue. For this reason, researchers have attempted to adapt Optical Coherence Tomography (OCT) and other coherence microscopies for use with internal imaging techniques that employ coherent fiber bundles. Unfortunately, however, coherent fiber bundles have proven unsuitable for coherence imaging, mostly because fiber bundle filaments are typically multimode waveguides which scramble the coherence signal, leak light between the filaments, and induce modal dispersion.
In OCT and optical coherence microscopy (OCM), as typically practiced, an object is illuminated with a focused beam of polychromatic light. By measuring the interferometric cross-correlation between the light backscattered from the object (the signal beam), and a reference beam, the time delay to various scattering features in the object may be inferred. In optical coherence domain reflectometry (OCDR), the relative time delay between the reference and signal beams is varied to measure the cross-correlation. In optical frequency domain reflectometry (OFDR), the frequency of the illumination is varied and the intensity of the interference is measured. A single beam can be scanned transversally to scan through an entire volume to create a 3-D image of the volume.
In conventional OCT or OCM, a single focused beam is scanned through the object to create the three-dimensional image. Unfortunately, this is not suitable for imaging with a fiber bundle. Because the fiber bundle consists of discrete imaging channels rather than a continuous space, the beam cannot be scanned to any arbitrary location in space and be confined in a fiber. This presents problems for conventional beam scanning apparatus because it is designed to scan the beam continuously over the object, usually in a raster-like pattern. Most of the time, the beam will not illuminate the core of a fiber and no useful signal will be recorded. To avoid having to scan the beam in a complicated and error prone pattern which would successively illuminate each fiber core, it is desirable that scanning the beam be avoided altogether.